Friday, November 07, 2008

Never Let Your Gas Tank Get Below Half Full

Friday's Urban Legend: True

How many of you have heard the story that you should always drive your car with the gas tank as full as possible? According to many, you should never let the gas in your tank fall below the half full mark on your gas gauge.

You probably thought this was a tale told by elderly wives—with apologies to old men who also tell tales.

Well, it turns out that there is actual, scientific, evidence to support this warning. Cliff Allen did the experiment according to the Sept. 13, 2008 issue of New Scientist [Petrol Gauge Challenge].

SLIGHTLY more practical routes to fuel economy occurred to Cliff Allen when he noticed that, according to his petrol gauge, the fuel in the top half of his tank lasted considerably longer than the bottom half. As any Feedback reader (and possibly only a Feedback reader) would, he investigated. Systematically.

Over several months he recorded the distances travelled using the fuel from the top and bottom halves. The average for the top was 400 kilometres (250 miles) and for the bottom a mere 300 kilometres (185 miles). Since then, he writes, "of course I have only used the top of my tank and have consistently achieved around 250 miles - I'm not stupid!"

Cliff was obviously keen to discuss this, at length, with his learned friends, "some of whom gained General Certificate of Secondary Education qualifications" at age 14. He was "mostly appalled at their incredulity and lack of interest".

However, his friend Alan suggested that the fact that petrol always comes out of the bottom of the tank causes it to use more petrol so we might benefit from turning the tank upside-down. John suggested the increased efficiency might be due to the height of the fuel, so the tank should be put on the roof. Mostyn proposed putting a brick in the tank, as this apparently works very well for saving water in toilet cisterns. Tony wants to make the top of the tank larger than the bottom, to increase the proportion of its volume at the top, and thinks a carrot shape would be optimal.

12 comments:

1) I've noticed that the "top half" of the tank appears to last longer than the bottom. I assume this is because the fuel gauge reading is not linear w.r.t. the actual volume in the tank, due to tank geometry and/or configuration of the linkage that translates float level into the electrical signal that drives the gauge.

2) My father always said to keep the tank topped up *in winter*. The theory is that in the cold, condensation can form in the empty volume, leading to water in the gas and possible icing in the lines. However, most brands now seem to add alcohol to their gas in winter, which acts as an antifreeze (ie. by making any water miscible with the gasoline). They used to sell "gas-line antifreeze" (isopropanol IIRC) for this purpose.

Since I'm talking to a biochemist, you can tell me if that last bit is plausible ;-).

1) I've noticed that the "top half" of the tank appears to last longer than the bottom. I assume this is because the fuel gauge reading is not linear w.r.t. the actual volume in the tank, due to tank geometry and/or configuration of the linkage that translates float level into the electrical signal that drives the gauge.

It was Allen's 5-year-old son who pointed out that the effect might be due to (we paraphrase) non-linearity of the petrol gauge. Where does he get such notions?

I'd also like to note that, from my experiences, the guages are configured such that "empty" actually means "you can drive another 50-100 km".

I had heard a plausible reason to not let the fuel tank get TOO close to bone-dry-empty: particulate matter in the gasoline.

Basically, the idea is that gasonline contains a tiny amount of grotty, particulate matter that can cause some mechanical troubles if too much of it gets into the combustion chambers at the heart of an engine. Running a fuel tank dry ensures that all of these particles get sucked into the gas line and into the engine, where they can cause some trouble. Keeping the gas tank at least half full means these particles are suspended in a large volume of liquid, and never reach high concentrations that can cause engine damage.

The obvious answer to this rather silly urban legend is: "gas gauges are inaccurate".

I've long noticed that my mileage is much better when the tank is more than half full. But the other day, I drove a company car that does even better--at the end of the day, the needle was near Empty, but when I started the next day, it was back up to about 1/8.

My car has one of those electronic computer thingies... you can set it to show fuel consumption, average speed or range (miles of travel remaining before the tank is empty) etc.

If I have been driving around town in traffic and have half a tank of fuel left, the range is around 240 miles. If I then start a long motorway drive at a steady speed the range gradually increases to around 280 miles after 30 miles of driving. If only the tank was actually filling up...

As to the original legend I suspect that with even a linear gauge the top half will last longer because most people will fill the tank until the pump cuts off (in the UK pumps stop automatically) and this is when fuel is in the filler tube - beyond the nominal 'full' point that the gauge is calibrated for.

The test is easy if you drive any distance to work. With the tank filled to the top drive to work and back to fill up again. Record the gallons used. Use the same pump and drive the same route to establish a conststant gallons used. Once you have the gallons, set the tank at 1/2, drive the same route, using the same pump put that amount of gallons in. keep doing this to see if the gas level goes up or down. With everything equal it should always go back to 1/2.

Well I have learned that is has to do with the rate of evaporation. If the tank is half or more empty that means that is there is more room in the tank for the gas to evaporate more rapidly. And unfortunately for us all the gas pump was not designed to such fumes...only liquid. So by aiming to keep the tank as full as possible you actually get to use more of your money's worth in GAS!!

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Principles of Biochemistry 5th edition

Disclaimer

Some readers of this blog may be under the impression that my personal opinions represent the official position of Canada, the Province of Ontario, the City of Toronto, the University of Toronto, the Faculty of Medicine, or the Department of Biochemistry. All of these institutions, plus every single one of my colleagues, students, friends, and relatives, want you to know that I do not speak for them. You should also know that they don't speak for me.

Superstition

Quotations

The old argument of design in nature, as given by Paley, which formerlyseemed to me to be so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.

Charles Darwin (c1880)Although I am fully convinced of the truth of the views given in this volume, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory.

Charles Darwin (1859)Science reveals where religion conceals. Where religion purports to explain, it actually resorts to tautology. To assert that "God did it" is no more than an admission of ignorance dressed deceitfully as an explanation...

Quotations

I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.

The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies raw material only. Natural selection directs the course of evolutionary change.

Rudyard Kipling asked how the leopard got its spots, the rhino its wrinkled skin. He called his answers "just-so stories." When evolutionists try to explain form and behavior, they also tell just-so stories—and the agent is natural selection. Virtuosity in invention replaces testability as the criterion for acceptance.

The first commandment for all versions of NOMA might be summarized by stating: "Thou shalt not mix the magisteria by claiming that God directly ordains important events in the history of nature by special interference knowable only through revelation and not accessible to science." In common parlance, we refer to such special interference as "miracle"—operationally defined as a unique and temporary suspension of natural law to reorder the facts of nature by divine fiat.

Quotations

My own view is that conclusions about the evolution of human behavior should be based on research at least as rigorous as that used in studying nonhuman animals. And if you read the animal behavior journals, you'll see that this requirement sets the bar pretty high, so that many assertions about evolutionary psychology sink without a trace.

Jerry Coyne
Why Evolution Is TrueI once made the remark that two things disappeared in 1990: one was communism, the other was biochemistry and that only one of them should be allowed to come back.

Sydney Brenner
TIBS Dec. 2000
It is naïve to think that if a species' environment changes the species must adapt or else become extinct.... Just as a changed environment need not set in motion selection for new adaptations, new adaptations may evolve in an unchanging environment if new mutations arise that are superior to any pre-existing variations

Douglas Futuyma
One of the most frightening things in the Western world, and in this country in particular, is the number of people who believe in things that are scientifically false. If someone tells me that the earth is less than 10,000 years old, in my opinion he should see a psychiatrist.

Francis Crick
There will be no difficulty in computers being adapted to biology. There will be luddites. But they will be buried.

Sydney Brenner
An atheist before Darwin could have said, following Hume: 'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist

Richard Dawkins
Another curious aspect of the theory of evolution is that everybody thinks he understand it. I mean philosophers, social scientists, and so on. While in fact very few people understand it, actually as it stands, even as it stood when Darwin expressed it, and even less as we now may be able to understand it in biology.

Jacques Monod
The false view of evolution as a process of global optimizing has been applied literally by engineers who, taken in by a mistaken metaphor, have attempted to find globally optimal solutions to design problems by writing programs that model evolution by natural selection.